This invention relates to an exposure apparatus having a light source of a short wavelength such as ultraviolet region and being arranged so that a gas flows around an optical element. Also, it relates to a device manufacturing method using such an exposure apparatus.
As regards light sources of exposure apparatuses, shortening the wavelength of light has been required. Currently, light of a standard ultraviolet region or light of a vacuum ultraviolet region is used. Generally, as the wavelength of light becomes shorter, the optical energy thereof becomes gradually larger. For example, as regards the photon energy of excimer lasers, it is 477.6 kJ/mol for a KrF laser (248 nm), 616.2 kJ/mol for an ArF laser (193 nm) and 753.9 kJ/mol for an F2 laser (157 nm). In these wavelength regions, there occurs light absorption in various matters. Further, there may occur decomposition or composition of substances due to a photochemical reaction. Thus, substances being present inside the apparatus may be deposited on an optical element to cause light absorption, or matters produced by a photochemical reaction may be deposited on an optical element to deteriorate the optical characteristic thereof. Further, any impurities mixed in a gas along an optical path may function to deteriorate the optical characteristic.
As regards problems which are attributable to those substances contained in an environment of optical elements, conventionally, a filter is used to improve the purity of a gas to be introduced or, alternatively, a gas of a higher purity is supplied, to thereby prevent deposition or to avoid absorption of light by impurities.
The contamination of the surface of an optical element in an exposure apparatus causes degradation of the optical characteristic thereof. The deterioration of the optical characteristic due to such surface deposition of the optical element becomes serious as the wavelength of the light source becomes short. This is for the following reasons. First, even if matter which is deposited on an optical element is not influential to the optical characteristic thereof in a range from the visible region to the standard ultraviolet region, it may function to absorb light of a shorter wavelength and may become adversely influential to the optical characteristic. Second, due to an increased photon energy of the light source, a photochemical reaction in which matter present along the optical path is involved is activated such that a reaction product is adhered to and deposited on the optical element to cause contamination of the same. Further, as regards matter mixed in a gas along the optical path, the matter may function as an impurity to cause degradation of the optical characteristic, or a photochemical reaction may produce impurities causing degradation of the optical characteristic. Thus, when the wavelength of a light source is shortened, many organic compounds which have not raised problems become impurities that cause deterioration of the optical characteristic.
Conventional filters for removing impurities function to attract impurities at a gas inlet of the apparatus to thereby remove them. As regards such filters, a filter which uses an ion exchange resin, activated charcoal or zeolite may be used. Alternatively, a chemical filter having acid matter or alkaline matter attached may be used. These filters have known properties, and a single filter does not attract and remove all the impurities. Thus, an optimum filter may be selected and used in accordance with a substance to be removed. If, therefore, there are different substances to be removed, optimum filters corresponding to these substances should be used in combination. This means that, if there are many varieties of impurities to be removed and they have different properties, a lot of optimum filters corresponding to these properties should be used. Thus, it is necessary to use many filters in combination.
In order to meet the problem of impurities described above, it is necessary to take measures not only for impurities contained in a gas from a supply source and for impurities resulting from a degassed component produced from a member during passage through a gas supply unit, but also for impurities resulting from a degassed component produced from a member inside and an optical unit or the like. As regards phthalic acid esters such as dibutyl phthalate (D.B.P.) or dioctyl pbthalate (D.O.P.), for example, if they are present in an ambience of an apparatus as being degassed from a member used, they are adhered to and deposited on an optical element because of their strong absorptivity with respect to light of a short wavelength. Although these substances are organic compounds having a relatively small volatility, since they are used widely as a plasticizer, they will easily become impurities in the atmosphere.
As regards matters deposited on an optical element, a photo-washing process for removing matters deposited on an optical element by an optical decomposition reaction based on light irradiation may be done. It has an additional effect of elimination by heating, or an effect of acceleration of a decomposition reaction as oxygen is changed to ozone or active oxygen. However, all the substances upon the optical element are not removed. There may be substances remaining on the optical element Also, to the contrary, matter mixed in an ambient gas may react by the irradiation with light, and reaction products may be deposited. As a matter of course, it is very difficult to remove contamination substances which have formed a mechanical connection with the surface of an optical element, and also it is a problem to remove substances adhered to or deposited on the optical element.
It is an object of the present invention to provide an exposure apparatus and/or a device manufacturing method using the same, by which adhesion of organic compounds to an optical element is prevented without a necessity of using many filters in combination.
It is another object of the present invention to provide a process or an arrangement effective to assure that no organic compound is present in a gas which flows along the surface of an optical element.
In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: an optical element; a gas supplying unit for supplying a predetermined gas around said optical element; and an organic compound decomposition mechanism for decomposing and removing an organic compound in the gas.
Preferably, the apparatus may further comprise a filter for removing a substance produced by the decomposition of the organic compound.
The apparatus may further comprise a chamber for accommodating therein a structure to allow the flow of the gas, wherein said organic compound decomposition mechanism may be disposed at at least one of an inlet port of the gas from said gas supplying unit to said chamber and a vicinity of said optical element.
The organic compound decomposition mechanism may decompose an organic compound by an electric discharging process.
The organic compound decomposition mechanism may decompose an organic compound on the basis of ionized atoms produced by electric discharging such as plasma discharging and corona discharging.
The apparatus may further comprise a filter for removing a substance produced by the decomposition of the organic compound based on the discharging process.
The apparatus may further comprise a filter for removing H2O produced by the discharging process.
The organic compound decomposition mechanism may include a fan for introducing the gas and a flow gauge for monitoring a flow rate of the gas.
The organic compound decomposition mechanism may adjust said fan on the basis of an output of said flow gauge.
The organic compound decomposition mechanism may include a catalyst for decomposing ozone.
The gas may be circulated through said organic compound decomposition mechanism.
In accordance with another aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: preparing an exposure apparatus as recited above; and performing an exposure process by use of the exposure apparatus while decomposing an organic compound by use of an organic compound decomposition mechanism of the exposure apparatus.
The method may further comprise removing a substance produced by the decomposition of the organic compound, by use of a filter.
In the present invention, an organic compound decomposition mechanism may be provided to decompose and remove an organic compound in a gas on the basis of an electric discharging process. This eliminates the necessity of using many filters in combination. Only a few filters for removing substances produced by the decomposition may be necessary. If the substances produced by the decomposition can be disregarded, these filters may be omitted.
The organic compound decomposition mechanism may be provided in the vicinity of an optical element. With this structure, a degassed component from a member inside the apparatus can be removed, and it can be assured that no organic compound is present in a gas flowing along the surface of the optical element. As a result of it, degradation of the optical performance of the optical element can be prevented effectively.